How can one get rid of the obsessive whispers (waswās) that affect wudu (ablution) and salah (prayer), making it difficult to distinguish between the obsessive whispering and the action required by Sharia?

The cure for obsessive thoughts is to ignore them and pay them no mind. When performing ablution, wash the limb once, twice, or thrice, and do not exceed that. When praying, say the takbir for consecration without hesitation, and recite the dhikr and verses naturally without repetition. If you doubt a recitation or the number of prostrations, disregard the doubt and pay it no heed. Continue this until Allah grants you healing.